The Klebsiella pneumoniae complex comprises seven K. pneumoniae- related species, including K. variicola . K. variicola is a versatile bacterium capable of colonizing different hosts such as plants, humans, insects and animals. Currently, K. variicola is gaining recognition as a cause of several human infections; nevertheless, its virulence profile is not fully characterized. The clinical significance of K. variicola infection is hidden by imprecise detection methods that underestimate its real prevalence; however, several methods have been developed to correctly identify this species. Recent studies of carbapenemase-producing and colistin-resistant strains demonstrate a potential reservoir of multidrug-resistant genes. This finding presents an imminent scenario for spreading antimicrobial resistant genes among close relatives and, more concerningly, in clinical and environmental settings. Since K. variicola was identified as a novel bacterial species, different research groups have contributed findings elucidating this pathogen; however, important details about its epidemiology, pathogenesis and ecology are still missing. This review highlights the most significant aspects of K. variicola , discussing its different phenotypes, mechanisms of resistance, and virulence traits, as well as the types of infections associated with this pathogen.

Klebsiella variicola is considered an emerging pathogen in humans and has been described in different environments. K. variicola belongs to Klebsiella pneumoniae complex, which has expanded the taxonomic classification and hindered epidemiological and evolutionary studies. The present work describes the molecular epidemiology of K. variicola based on MultiLocus Sequence Typing (MLST) developed for this purpose. In total, 226 genomes obtained from public data bases and 28 isolates were evaluated, which were mainly obtained from humans, followed by plants, various animals, the environment and insects. A total 166 distinct sequence types (STs) were identified, with 39 STs comprising at least two isolates. The molecular epidemiology of K. variicola showed a global distribution for some STs was observed, and in some cases, isolates obtained from different sources belong to the same ST. Several examples of isolates corresponding to kingdom-crossing bacteria from plants to humans were identified, establishing this as a possible route of transmission. goeBURST analysis identified Clonal Complex 1 (CC1) as the clone with the greatest distribution. Whole-genome sequencing of K. variicola isolates revealed extended-spectrum β-lactamase- and carbapenemase-producing strains with an increase in pathogenicity. MLST of K. variicola is a strong molecular epidemiological tool that allows following the evolution of this bacterial species obtained from different environments.

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An antibiotic-susceptible and hypermucoviscous clinical isolate of Klebsiella variicola (K. variicola 8917) was obtained from the sputum of an adult patient. This work reports the complete draft genome sequence of K. variicola 8917 with 103 contigs and an annotation that revealed a 5,686,491-bp circular chromosome containing a total of 5,621 coding DNA sequences, 65 tRNA genes, and an average G+C content of 56.98%.

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A colistin-resistant mcr-1-carrying Escherichia coli strain, RC2-007, was isolated from a swine farm in Mexico. This extraintestinal and uropathogenic strain of E. coli belongs to serotype O89:H9 and sequence type 744. Assembly and annotation resulted in a 4.9-Mb draft genome that revealed the presence of plasmid-mediated mcr-1-ISApI1 genes as part of a prophage.

A clinical isolate of extended-spectrum-β-lactamase-producing Klebsiella quasipneumoniae subsp. similipneumoniae 06-219 with hypermucoviscosity phenotypes obtained from a urine culture of an adult patient was used for whole-genome sequencing. Here, we report the draft genome sequences of this strain, consisting of 53 contigs with an ~5.6-Mb genome size and an average G+C content of 57.36%. The annotation revealed 6,622 coding DNA sequences and 77 tRNA genes.